Internal bulbose or leaf-shaped organ that is part of the water vascular system and directly underlies tube-feet. It acts as a fluid resevoir for expansion and constraction of the associated tube-foot.
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anal (fasciole)

A fasciole band that runs vertically from the subanal fasciole passing on either side of the periproct (ppt).
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anal lunule

A lunule that pierces interambulacrum 5 and which, on the adoral face, is closely associated with the periproct.
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anal sulcus

A groove on the aboral surface running towards the posterior of the test from the periproct.
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anal tuft

A single or double cone of spines that projects from the rear of the test beneath the anus. These spines are used in constructing the subanal funnel and are surrounded by a subanal fasciole.
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anisopore

The connection between tube-foot and internal water vessel where there are a pair of pores of markedly unequal size.
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The small area of plates forming the aboral surface of the test at the apex of the ambulacral and interambulacral zones. Composed of ocular and genital plates, which (in regulars) surround the periproct.
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apophysis

Interambulacral element of the perignathic girdle. An internal flange (ap) that originates from the most adoral interambulacral plates. Lantern protractor muscles attach to these. [Picture shows the interior of the oral region.]
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arbaciid plate compounding

A compound ambulacral plate composed of three (rarely four) elements of which the middle is always the largest. Upper and lower elements are always small and do not reach the perradius.
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areole

A flat platform of fine stereom surrounding the boss and marking the attachment site of spine muscle on a tubercle. Can be depressed in cidaroids
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